1. Field of Invention
The present invention relates to a joint restraint assembly. More particularly, the present invention relates to a joint restraint assembly for connecting pipes together, or to other objects.
2. Description of Related Art
Joint restraint assemblies of several types are known in the art and which comply with pipe connection standards, such as the ANSI/AWWA C111/A21.11, entitled “American National Standard for Rubber-Gasket Joints for Ductile-Iron Pressure Pipe and Fittings.” A conventional restraint assembly comprises an annular body having a plurality of threaded bolts equally spaced around the body, with the threaded bolts extending through threaded bores disposed through the body along radial lines, or radial lines inclined at an angle of less than 90 degrees from the pipe axis. The end of each bolt is configured to directly bear on the pipe surface or another component that, in turn, bears on the pipe surface. The head of each bolt typically includes a torque head with a feature that is designed to shear when a predefined torque is applied to the bolt. The shear feature is intended to result in the application of a specific torque to each bolt without the use of a torque measuring wrench.
Once the joint restraint assembly is positioned adjacent the end of a pipe, the pipe end is mated into the socket and then the flange portion of the joint restraint is secured to a corresponding flange portion on the socket side via flange-connecting fasteners (e.g., “T-bolts”). See, for example, U.S. Pat. Nos. 3,333,872 (Crawford, Sr. et al.); 3,726,549 (Bradley, Jr.); 4,848,808 (Pannell et al.) and 4,779,900 (Shumard). It should be understood that subsequent reference to “bolt” hereinafter refers to the bolts used to secure the joint restraint assembly to the pipe end and not to the flange-connecting fasteners used to secure the flanges together unless indicated.
In some conventional restraint assemblies, the end of each bolt is either configured to penetrate the surface of the pipe or to bear upon a pad, clamping block, or segment with one or more gripping teeth to penetrate the surface of the pipe. In conventional restraint assemblies that utilize segments with one or more gripping teeth to penetrate hard pipe materials, such as ductile iron, the length of the gripping teeth penetrating the pipe surface and the depth of the penetration are limited by the force produced as a result of applying the specified torque to the threaded bolt. As a result, a conventional restraint assembly applies loading that is localized at the positions of the threaded bolts or clamping blocks. The allowable mechanical or pressure loading, tending to pull the pipe out of the restraint assembly, is limited by the shear strength of the pipe material and the area of the pipe material that would have to shear in order to permit the penetrating gripping teeth to slip along the pipe surface, with that shear area being related to the penetration depth of the gripping teeth into the pipe material and the circumferential length of the penetration. The allowable mechanical or pressure loading, tending to pull the pipe out of the restraint assembly, is also limited by the shear strength of the segment material and the area of the segment gripping teeth that would have to shear in order to separate the gripping teeth from the segment, with that shear area being related to the penetration depth of the gripping teeth into the pipe material and the circumferential length of the penetration. Accordingly, conventional restraint assemblies are often inadequate for comparatively high levels of mechanical loading and/or pipe internal pressure. See, for example, U.S. Pat. Nos. 817,300 (David); 3,333,872 (Crawford, Sr. et al.); 3,726,549 (Bradley, Jr.); 4,397,485 (Wood); Des. 294,384 (Endo et al.);
One type of conventional restraint assembly in the prior art comprises an annular body having a plurality of cavities adjacent to the pipe with a clamping block configured to fit into each cavity. See, for example, U.S. Pat. Nos. 4,092,036 (Sato et al.); 4,779,900 (Shumard); 4,896,903 (Shumard) and 5,071,175 (Kennedy, Jr.). A plurality of bolts equally spaced around the body, disposed through the body along radial lines inclined at an angle of less than 90 degrees from the pipe axis, that extend through non-threaded oval holes into the cavities. The inboard surface of each cavity is perpendicular to the axis of the oval hole such that it is inclined to the axis of the pipe. Each bolt is configured with an integral annular flange, or collar, that is slidably in contact with the inclined surface of the cavity, and the threaded shank of each bolt is engaged in a threaded hole in the corresponding clamping block. Each clamping block is configured with teeth to directly bear on the pipe surface. When the threaded bolt is turned, force is applied to the clamping block causing the teeth to dent the pipe surface. An equal and opposite force is applied to the contact between the integral annular flange of the bolt and the inclined surface of the cavity. These contact surfaces are not polished and lubricated, and they are usually covered with a protective coating. (See for example EBAA Iron Sales, Inc., Wedge Action Megalug™ Field Installed Joint Restraint; EBAA Iron Sales, Inc., Series 3000 Multi-Purpose Wedge Action Restraint; or EBAA Iron Sales, Inc., Series 2000PV Megalug™ Retainer Glands for PVC Pipe with Cast-Iron or I.P.S. Outside Diameters with M. J. Bells). The service environment ordinarily results in corrosion, sediment particulate and other conditions that do not permit a low coefficient of sliding friction between the integral annular flange of the bolt and the inclined surface of the cavity.
The concept of this “wedge” type of restraint assembly is that as the mechanical or pressure loading tends to pull the pipe out of the restraint assembly, the annular flange on the threaded bolt slides along the inclined surface of the cavity causing the clamping block teeth to dent the pipe surface more deeply, thereby resisting the tendency of the pipe to pull out of the restraint assembly. In practice, the frictional forces that resist sliding between the annular flange on the bolt and the inclined surface of the cavity are proportional to the force being applied to dent the pipe and, in combination with the mis-alignment of the force vectors tending to cause the annular flange of the bolt to bind instead of slide, the theoretical effect is only partially realized. All of the “wedge” type prior art has this inherent characteristic which limits its effectiveness. One manufacturer of this type of joint restraint assembly explains in its publications that this type of restraint works without wedge movement to resist normal operating pressure in the pipe, but wedge movement responds “only as the external force is increased” from additional external conditions such as subsidence, waterhammer, traffic loads or small tremors (EBAA Iron Sales, Inc., Wedge Action Megalug™ Field Installed Joint Restraint). However, in the normal operation of this type of restraint assembly, the force generated by the bolt is applied to the clamping block causing its teeth to dent the pipe surface, and if the wedge effect is able to overcome its inherent sliding friction and binding characteristics, it does so, inefficiently, only under additional external loading conditions.
Another type of conventional restraint assembly in the prior art comprises an annular body with equally spaced cavities with a segment configured to fit into each cavity. A threaded bolt extends through a threaded bore into each cavity, and the force generated by the threaded bolt is applied to the segment to cause the edges of the segment to dent the pipe surface. The end of the threaded bolt is manufactured with a hemispherical form, and it fits into a dished socket in the segment. (See Sigma/Napco, SuperLug™, Pipe Restraints for Ductile Iron Pipe). The manufacturer's publications state that: the ball and socket allows lug deflection at any angle, thereby allowing the lugs to “rock”, actually gripping the pipe more securely as pressure-induced load increases; and pressure-induced load causes the primary contact edge of the SuperLug™ teeth to “grab” the pipe surface, further increasing pressure-induced load restraint.
However, testing of this design in larger sizes, such as for 30 to 48 inch diameter pipes, revealed that the force of the radial threaded bolt, even in combination with the “rocking” or “cam action”, was insufficient to adequately grip the pipe, and other undesirable effects occurred such as bending of the threaded bolt. A parametric analysis of the design revealed that the pressure-induced load, tending to pull the pipe out of the restraint assembly, overpowered the capability of the design as the pressure-induced load increased with the square of the pipe diameter. Using a restraint assembly for 48 inch pipe as an example, the total axial load is in excess of 1,000,000 pounds during the hydrostatic proof test at 500 psi pressure. This requires each of 32 segments, and the threaded bolt forcing it into the surface of the pipe, to resist almost 32,000 pounds of pressure-induced load. When tightened to the specified torque, the threaded bolt is capable of applying 7,500 pounds of force to the segment, causing its edges to dent the surface of the pipe, but the indentation was not of sufficient depth and circumferential length to resist the 32,000 pounds of pressure-induced load. With the known shear strength of the pipe material, the required shear area of the pipe material that would have to resist shearing in order to prevent the penetrating, gripping teeth from slipping along the pipe surface, requires both a greater depth of penetration into the pipe material and a greater circumferential length of that penetration.
It would be desirable for a restraint assembly to overcome the inherent problems and limitations in the prior art and reliably accommodate comparatively high levels of mechanical loading and/or pipe pressure, and to do so without relying upon the limited force produced by the application of the specified torque to the threaded bolts.
The entire disclosures of all of the references cited herein are incorporated by reference.